1. Field of the Invention
The present invention relates to a piezoceramic valve actuator sandwich assembly, and particularly, to a valve incorporating such an actuator.
2. Description of Related Art
Piezoceramic elements are elements in which the application of a voltage between certain faces of the piezoceramic element produces a mechanical distortion of the piezoceramic element. Piezoceramic valves incorporate a piezoceramic element in a valve actuator assembly and take advantage of the properties of the piezoceramic element to open and close a valve, thereby controlling gas and fluid flow through the valve.
Piezoceramic valves offer several advantages relative to electromagnetic valves. The primary advantage is that the piezoceramic valve converts only a small amount of power to wasted heat as compared to electromagnetic valves and therefore, consumes much less power. The power consumption of the piezoceramic valve may even be so low as to be negligible when compared to the required support circuitry.
In a conventional piezoceramic valve, a piezoceramic element is attached to a flexible metal substrate. The piezoceramic element can be of a unimorph type which has only a single piezoceramic layer, or a bimorph type which has two piezoceramic layers, one each attached to opposite sides of a flexible plate. Both types of piezoceramic elements are conventionally available. The piezoceramic element/substrate assembly is fixed to a valve body such that a portion of the assembly can cover and uncover a valve orifice in the valve body as the piezoceramic element is energized and deenergized, or vice-versa. The piezoceramic element/substrate assembly can be fixed to the valve body in the form of a cantilever beam or a diaphragm. In one known embodiment, as shown in U.S. Pat. No. 4,610,426, a circular piezoceramic element/substrate assembly operates as a diaphragm to cover a valve seat when the piezoceramic element is in a deenergized state and to deflect away from the valve seat to open the valve seat when the piezoceramic element is energized.
One problem with a conventional valve as shown U.S. Pat. No. 4,610,426, is that the sealing force of the piezoceramic valve actuator against the valve seat is limited due to the limited deflection force the piezoceramic bending element can generate. Therefore, the valve can only be used in low pressure situations, since higher pressures would merely push the valve actuator away from the valve seat, resulting in leakage. This leakage is best prevented by providing two valve chambers in the valve, one each isolated on opposing sides of the valve actuator diaphragm. The valve seat and orifice are provided in one of the chambers and the other chamber is exposed to the inlet pressure of the valve, thereby providing an equalizing pressure on the back side of the diaphragm that offsets the pressure on the diaphragm at the valve seat. Since the pressure on both the valve side and the back side of the diaphragm is equalized, only a small force is needed to seal the valve seat, even when the inlet pressure is relatively high.
Another problem with conventional piezoceramic valves is the isolation of the piezoceramic element from the gas or fluid flow through the valve. First, it is desirable to electrically insulate the electrically active piezoceramic element from the gas or fluid flow. Further, many gases and fluids are corrosive and the valve actuator must be insulated to provide protection from chemical attack. Additionally, low molecular weight gases, such as hydrogen or helium, are able to diffuse through the porous ceramic of the piezoceramic element, resulting in leakage of the gas through the valve. In a high pressure valve utilizing a normalizing back pressure chamber as described above, the piezoceramic element must be isolated from the gas or fluid flow not only its valve side but also on its back side. This must be done while providing a mounting for the diaphragm that not only sealingly isolates the piezoceramic element from the gas or fluid, but also provides enough flexibility for the diaphragm to deflect and operate properly.